1. Field of the Invention
The present invention relates to a heat exchanger of an air conditioner, and more particularly to a heat exchanger of an air conditioner in which grille type grilles are formed in a flat fin.
2. Description of the Prior Art
A heat exchanger of an air conditioner according to the prior art includes, as illustrated in FIG. 1, a plurality of flat fins 1 provided in parallel at a predetermined spacing, and heat transfer pipes 2 arranged perpendicular to the fins 1 and provided in zigzag style. The air current moves between the fins 1 in a direction represented by an arrow to perform a heat exchange with fluid in the heat transfer pipes 2.
Furthermore, as regards heat fluid characteristics around the fins, a temperature boundary layer 3 on a heat transfer surface of the fins 1 becomes, as depicted in FIG. 2, thicker in proportion to the square root of a distance from an entry end of the air current. Thus, there is a disadvantage in that a heat transfer rate between the air current and fins is remarkably decreased as the distance from the entry end of the air current increases, and thereby decreases the heat transfer efficiency of the heat exchanger.
There is still another disadvantage when air current of low speed moves past the heat transfer pipes 2, in that a cavitation zone 4 is generated at a rear area of each heat transfer pipe 2 (i.e., the area cross hatched in FIG. 3), so that a heat transfer rate is remarkably decreased at the cavitation zone 4 to thereby cause a drop in heat transfer performance of the heat exchanger.
As a prior art to deal with the aforementioned disadvantages, Japanese Laid/Open Utility Model Application No. Sho. 55-110995 is disclosed, where the fins of an air conditioner are, as illustrated in FIG. 4, formed with groups of slit-forming grilles 5a, 5b, 5c, 5d, 5e and 5f disposed in a vertical gap formed between the heat transfer pipes 2.
In other words, the grilles 5a, 5c and 5e as illustrated in FIG. 5, are caused to protrude from one surface of the fin 1 by a cutting and bending process, with the grilles arranged at a predetermined interval, and the other grilles 5b, 5d and 5f protrude from the opposite surface of the fin 1 and are arranged between the grilles 5a, 5c and 5e.
The fins 1 formed with the groups of grilles 5a, 5b, 5c, 5d, 5e and 5f can expect a high heat transfer performance compared with fins having no grilles. The upstream grilles 5a and 5b provide a high heat transfer performance because a thin temperature boundary layer is formed thereby. However, the heat transfer performance becomes lower at the downstream grilles 5c, 5d, 5e and 5f because those grilles 5c, 5d, 5e and 5f are disposed in the temperature boundary layer formed by the grilles 5a and 5b.
There is another problem in that a cavitation zone where the air current does not flow is generated at a rear side of the heat transfer pipe 2, thereby reducing heat transfer efficiency.
There is still another problem in that the fin has a limited heat transfer area and is relatively easily bendable.